The overall objective of this proposal is to define the metabolic consequences, interventions, and mechanisms of a novel signaling cascade involving retinoic acid receptor (RAR) and fibroblast growth factor 21 (FGF21) in non-alcoholic fatty liver disease (NAFLD), one of the most common chronic liver diseases in the United States. Hepatic steatosis is an early and reversible stage of NAFLD, which can advance to irreversible cirrhosis and hepatocellular carcinoma with no effective therapies. Therefore, the discovery of novel metabolic pathways that present therapeutic targets is required to effectively manage NAFLD. As the major metabolite of vitamin A, all-trans-retinoic acid (RA) is a natural ligand of retinoic acid receptor (RAR) and is clinically used for anti-cancer therapy. However, the role of different RAR isotypes in regulating liver physiology and NAFLD is a novel and unexplored area. While the newly discovered hepatocyte-derived hormone FGF21 is emerging as a potential therapeutic target for treating metabolic disease, the upstream regulators of FGF21 remain largely unknown. Our recent discovery of a previously unrecognized crosstalk between RAR and FGF21 has changed this view and leads to a series of novel and exciting data that represent the core of our specific aims. Specifically, our results show that 1) FGF21 gene expression is induced by RAR?r RAR?ut not by RAR?; 2) RA increases fatty acid oxidation at least partially through FGF21 in vitro; 3) Hepatic overexpression of RAR?y an adenoviral gene delivery improves hepatic steatosis and insulin resistance in diet-induced obese mice; and 4) Hepatic overexpression of RAR?lso results in increased FGF21 production in mice. To extend these exciting observations, the Central Hypothesis is that hepatic RAR functions as a transcriptional regulator of FGF21 to maintain hepatic lipid homeostasis during prolonged fasting and to attenuate the progression of NAFLD. There are three Specific Aims. 1) To determine whether hepatic RAR regulates FGF21 induction and lipid homeostasis under nutrient deprivation conditions. State-of-the-art approaches including gene expression profiling analysis, metabolomics and lipidomics analyses will be conducted to identify new regulators and lipid metabolites involving RAR and/or FGF21 signaling. 2) To determine whether hepatic RAR slows the development of NAFLD and insulin resistance through FGF21 using in vivo adenoviral gene transfer targeting each RAR isotype and FGF21-/- mice. 3) To elucidate the molecular basis of the integrated signaling of hepatic RAR and FGF21 in de novo lipogenesis under in vivo and in vitro conditions of metabolic stress. Overall, accomplishing this proposal will establish th physiological role of an RAR-FGF21 signaling in the regulation of lipid homeostasis. These innovative experiments are expected to identify new therapeutic targets for the management of NAFLD and its related metabolic disease.